In the testing for the mutagenic activity of environmental agents a clear preference is expressed for the dominant lethal method. Second to the dominant lethal method are in vivo or in vitro tests for the induction of chromosomal damage. In view of the now widespread application of the dominant lethal method and of the various tests for chromosomal damage in commercial testing laboratories, it seems appropriate to consider to what extent these methods are really diagnostic for detecting transmissable genetic damage in the form of gene mutations and viable translocations. Dominant lethals consist in part of open chromosome breaks and in part of asymmetrical exchanges. The fruitfly Drosophila is excellently suited to study open chromosome breaks (loss of a ring-shaped X-chromosome), chromosome exchanges (heritable translocations), and mutations (recessive lethals or visibles) in one and the same experiment. It is proposed, therefore, to undertake a systematic study of the frequencies with which these different categories of genetic damage are induced in a variety of different germ cell stages by representatives of various different groups of chemical mutagens under different modifying conditions, such as storage, chronic application, or chemical co-treatment. The outcome of such an investigation will be of value to decide whether results based on one easily detectable class of genetic damage, as dominant lethals, have more general validity. In addition, it is proposed to include tests to screen for the ability of the chemicals to induce small deletions, non-disjunction, and recombination. It is hoped that the data collected will expose and identify problems which need to be studied in depth in mammalian systems.